An increasing motor traffic in densely built-up areas has resulted in an increase in the number of car crashes and thereby also in an increase in the injuries to passengers. It has turned out that rear-end collisions at low speed often results in so-called whiplash injury. Such injuries have become one of the predominant problems in the automotive industry from the point of view of passenger's safety.
A theoretical survey of the causes for whiplash injuries revealed that such injuries mainly occur due to the momentum the head has during the movement of the vehicle caused by the collision. As soon as the vehicle collides, the head is being pushed back with a force that is proportional to the speed of the vehicle caused by the collision. The head hits the headrest as a result of this backward movement of the head and rebounds back with a force which is very much dependent on the resilience behaviour of the material which is being used in the headrest. In addition to the rebound force, the extent of head displacement after the rebound is also very important from the whiplash point of view.
A number of headrests claiming to alleviate whiplash injury have been disclosed in the past.
Thus, for instance, DE 197 46 413 A1 discloses a headrest comprising a support member fixed to a seat and a cushioning member of polyurethane foam attached to said support member and enclosed in a covering material. The cushioning member of this prior art headrest should be made of a material having a low rebound elasticity of 30% or lower, preferably 25% or lower. In an embodiment of the headrest a combination of two cushioning members are used, one having a rebound elasticity of 10% or lower and the other having a higher rebound elasticity in order to improve the feeling of the headrest. According to Table 1 rebound elasticity values of lower than 10% are obtained by means of polyurethanes having a density of from 160 kg/m3 to 195 kg/m3.
PCT WO95/05954 discloses a headrest including a cushion which is filled with a fluid and is equipped with one or more weakened areas or openings covered by a sealing arrangement. At a predetermined pressure against or in the cushion the fluid is displaced into a recipient outside the cushion with a defined fluid flow rate.
Another headrest based on the same basic idea of a cushion filled with a fluid which is displaced into a recipient outside the cushion on collision is disclosed by U.S. Pat. No. 3,680,912.